Radar apparatuses for collision avoidance are widely used particularly for luxury vehicles; however, costs and manufacturing hours are to be reduced in order to expand the use of radar to a number of vehicles. As an existing radar apparatus, a planar antenna module that integrates an antenna substrate and an integrated circuit (IC) substrate is available (see Japanese Patent No. 4803172 (International Publication Pamphlet No. WO 2006/098054), for example). In this module, a metal plate (made of aluminum or the like) having cavity waveguides formed therein is sandwiched between the antenna substrate and the IC substrate so that the IC substrate, the metal plate, and the antenna substrate are stacked, and through-transmission (transmission in the thickness direction of the substrates) is performed via waveguide transitions. This structure is simplified and illustrated in FIGS. 1A and 1B. FIG. 1A is a perspective view, and FIG. 1B is a cross-sectional view taken along line IB-IB of FIG. 1A.
By connecting a patch antenna (radiating element) 111 formed close to an IC substrate 101 to a patch antenna (radiating element) 113 formed close to an antenna substrate 103 via a waveguide 112 that passes through an aluminum plate 102, through-transmission of millimeter waves of about 60 to 76 GHz from a monolithic microwave (MM) IC to an antenna unit is performed. The waveguide 112 is used, and therefore, it is possible to perform through-transmission with a good isolation characteristic even if a plurality of radiating elements are disposed adjacent to one another.
However, even the simplified structure illustrated in FIGS. 1A and 1B uses two dielectric substrates and one aluminum plate, and alignment to be performed at the time of assembly is an obstacle to cost reduction. Furthermore, through-transmission is performed via waveguide transitions, which has a shortcoming, such as a large transmission loss. In the configuration illustrated in FIGS. 1A and 1B, in a case where the radiating elements 111 and 113 have dimensions of 1.0 mm×1.8 mm and the waveguide 112 has dimensions of 1.8 mm×3.4 mm×4.2 mm, for example, the transmission loss at 80 GHz is −5.2 dB. Therefore, a through-transmission technique using only dielectric substrates with a smaller transmission loss is anticipated.
Other related art is available in which a high-frequency line conductor is disposed in a position facing an electrode film, and an end portion of the line conductor is disposed so as to face a slot (opening) formed on the electrode film to thereby electromagnetically couple electromagnetic waves that propagate through the line conductor to a dielectric waveguide line disposed below (see Japanese Patent No. 3517148 (Japanese Laid-Open Patent Publication No. 2000-252712), for example). In this configuration, a group of sidewall through conductors (hereinafter referred to as “shield pins”) are disposed in a dielectric to thereby block electromagnetic waves and suppress the spread of an electric field. However, in order to attain a desired isolation characteristic, the shield pins have to be disposed at a certain interval in all regions where through-transmission is performed. In a typical substrate manufacturing method, shield pins are formed by drilling and inserting electrodes while performing alignment, after forming a stacked substrate, and therefore, costs increase as the number of shield pins increases.
The embodiments provide a stacked waveguide substrate excellent in transmission loss and isolation characteristics with a simple and low-cost configuration.